By way of example of all grab cranes, the operation of a dredge crane operating a grab having plural sides, such as a two sided clamshell bucket in underwater dredging, may be considered. Such involves the sequential steps of lowering, closing, raising, luffing out, opening, luffing in, and again lowering, etc. under changing bottom depths and flowing water. The three steps of lowering, closing and raising are the three along with braking that are the most critical, and are the most difficult to program either manually, or otherwise, because they occur out of sight of the crane operator. Even if visible through clear water over a range of depths it would be difficult to determine visually when the bucket has established a desirable working position with respect to the bottom and is ready for closing to pick up a load in a single pass without either "burying" the bucket or "skimping" the load. Objectionably, slack hoist lines from poor braking can permit the "burying" of a bucket and incur many kinds of delaying complications including the tilting or tumbling of the bucket, improper closing of the bucket, and, undue stress movements on the hoist.
Full automation for a wide range of changing depths and conditions is highly desirable for these three steps. However, confusing conditions are experienced because in the above sequence two non-contiguous steps occur when the bucket is the lightest on its hoist winch, namely, when the winch brake is applied for luffing and opening the bucket, and when the bucket engages the bottom and is closed. Also two non-contiguous steps occur when the bucket is the heaviest on the winch, namely, when being raised and when being lowered. In both, similarities between the alternative steps provide like conditions that are substantially indistinguishable for automated monitoring, and, any requirements or provisions for manual override for these steps involves delays and further complications that permits only partial automation. Additionally, two power efforts are involved and their coordination and control have to be transmitted to the bucket through separate connections additionally carried by the boom. One effort is to open and close the bucket, and the other effort is to raise and lower the bucket. Heretofore, these efforts have generally been controlled separately and manually with a substantial waste of operational time and power. Accordingly, time and labor saving operative automation has heretofore been limited.